SIP Call silence after sip re-invite after 15mins

Dear Fortinet support,

 I am the new here and get the problem on session helper in SIP call.

My story is like this.  my VoIP Gateway is NAT behind the fortinet. It is need enable the session helper transfer the SIP SDP message IP and RTP port to build the RTP Channel up.

But, the problem is , when the sip re-invite the message sent, default is 15mins , the RTP Port is changed after fortinet sent out via NAT. RTP Port is changed and it happen the RTP Channel is changed port. But my VoIP Gateway don't change the RTP Port.

It make the call silence because the RTP channel is lose.

voip gate (RTP Port : 12345) -> fortinet (RTP Port :34567) -> sip server (RTP Port :34567) -> PSTN

re-invite happen

voip gate (RTP Port :12345) -> fortinet (RTP Port :34577) -> sip server (RTP Port :34577) -> PSTN

RTP Channel is lose after fortinet changed the RTP Port.

Did anyone meeting have above problem? Thanks.